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Laboratory-scale investigation of the removal of
hydrogen sulfide from biogas and air using industrial
waste-based sorbents
Olumide Wesley Awe, Doan Pham Minh, Nathalie Lyczko, Ange Nzihou,
Yaqian Zhao
To cite this version:
Olumide Wesley Awe, Doan Pham Minh, Nathalie Lyczko, Ange Nzihou, Yaqian Zhao.
Laboratory-scale investigation of the removal of hydrogen sulfide from biogas and air using industrial
waste-based sorbents. Journal of Environmental Chemical Engineering, Elsevier, 2017, 5 (2), p.1809-1820.
�10.1016/j.jece.2017.03.023�. �hal-01619255�
Laboratory-scale
investigation
of
the
removal
of
hydrogen
sulfide
from
biogas
and
air
using
industrial
waste-based
sorbents
Olumide
Wesley
Awe
a,b,*
,
Doan
Pham
Minh
b,**
,
Nathalie
Lyczko
b,
Ange
Nzihou
b,
Yaqian
Zhao
aaDoogeCentreforWaterResourcesResearch,SchoolofCivilEngineering,UniversityCollegeDublin,Newstead,Belfield,Dublin4,Ireland bUniversite’deToulouse,MinesAlbi,CNRSUMR5302,CentreRAPSODEE,CampusJarlard,Albi,F-81013cedex09,France
Keywords: Biogas Activatedcarbon Calciumcarbonatewaste Purification
Adsorption H2Sremoval
ABSTRACT
Biogasisavaluablerenewableenergythatcanbeusedasafuelorasrawmaterialfortheproductionof hydrogen,synthesisgasandchemicals.ApartfromitsmainconstituentofCH4andCO2,italsocontained
variousundesirablecontaminants.TheremovalofthesecontaminantssuchasH2Swillsignificantly
improvethequalityofthebiogasforfurtheruse.Thisworkinvolvedthevalorizationofcalciumcarbonate (CaCO3)-basedsolidwastesfortheremovalofH2Sfromthebiogasstream. Thesolidwastes were
analyzedbydifferentphysicochemicalmethodsCaCO3wasfoundasthemaincomponentofbothsolid
wastes,whiletraceamountsofotherelementssuchasMg,Al,etc.werealsopresent.Thesolidwastes weredispersedinwaterandtheresultedsuspensionsweretestedfortheremovalofH2Sfromthegas
phase,usingatriphasicgas/liquid/solidglassreactoratroomtemperatureandpressure.Acommercial activatedcarbon(AC)andapurecommercialcalcitewerealsousedforcomparisonpurpose.Inaddition, mixturesofACwithCaCO3wasteindifferentratioswerealsotested.Theinfluenceofthematrixofgas
(airorbiogas)ontheremovalofH2Swasevaluated.Bothsolidwastesshowedhigherperformance
comparedtothepurecommercialcalciteforH2Sremoval.ThecouplingofaCaCO3wasteandACallowed
improvingthesorptionperformancecomparedtopurecommercialCaCO3andACusedalone.Theresults
obtainedopenanewpromisingwayforthevalorizationofCaCO3-basedwastesforH2Sremovalfrom
biogas.
1.Introduction
Biogasisarenewableenergyresourcethatcanbeanalternative
solutionfortheworldinsatiableenergydemandsandatthesame
time help in reducing waste and the greenhouse gas (GHG)
emissions[1].It isalsoregardedas carbonneutral becausethe
carbon in biogas comes from organic matter (feedstocks) that
captures this carbon from atmospheric CO2 over relative short
timescale[2]. Whenbiogas is produced, wasteis converted to
energythereby reducingtherefuse mountain(landfill)disposal
andprovidingwithenergysuppliessecuritywithoutany
signifi-cant environment impact [1]. The production of biogas also
provides an eco-friendly, high-value useable by-product
(bio-fertilizer) forsoil conditioningand improvement[2].Thus it is
highlyregardedaseffectivemethodofwastemanagement[3,4].
The chemical composition of biogas largelydepends on the
origins andthekindofsubstrates usedsuchas methanationof
biomass,organicwastesfromsewagesludge,animalfarmmanure,
energycrops,andagro-foodindustrywaste[5].Differentprocesses currentlyexistandcanbeusedforthegenerationofbiogassuchas
anaerobicdigestion,anaerobicco-digestion,landfills,commercial
composting,andagro-foodindustrydigestionfacilities,underboth
mesophilic(35!C)andthermophilic(55!C)conditions[6].
Thecompositionofrawbiogasfromtheanaerobicdegradation
ofsewagesludge,livestockmanure,industrialandagro-bio-waste
areshowninTable1.Thelowerheatingvalueofbiogasrangesfrom
15to30MJ/Nm3andislowerthanthatofnaturalgas,whichis
around36MJ/Nm3[5,6].
Hydrogen sulfide along with other S bearing compounds
(mercaptansetc.)arethemostcommoncontaminantsinbiogas
and their content, which can vary from 100 to 10,000ppm,
dependslargelyonthecompositionoftheorganicmatter,mostly
protein-rich streams. They are also formed in different
* Correspondingauthorat:DoogeCentreforWaterResourcesResearch,Schoolof CivilEngineering,UniversityCollegeDublin,Newstead,Belfield,Dublin4,Ireland.
** Correspondingauthor.
E-mailaddresses:wesley.awe@ucdconnect.ie(O.W.Awe),
anthropogenic processessuch as; coal combustion,papermills,
food industries, wastewater treatment plants, animal farms,
gasification of biomassor coal for syngasproduction or
petro-chemical processing, etc. [7–9]. So it can be used in chemical
processes for the production of various products, including
syntheticgas(syngas,mixtureofCOandH2),hydrogen,methanol
and otherhydrocarbons[10].Theymustberemovedbefore any
utilizationbecausetheyarehighlycorrosivetothemetallicpartsof
engines,pipes,pumps,compressors,gasstoragetanks,valvesand
reducethelifespanofprocessequipment,mostespeciallyinthe
presenceofwaterandtraceofoxygeninthebiogas.Thisresultin
extracostsofinfrastructuresandmaintenance[11,12].Theyhave
also serious environmental concerns due to their oxidation to
sulfur dioxide (SO2)and sulfuric acid(H2SO4)[2].The
environ-mentalconcernsofH2Sincludeacidrainduetohighsolubilityof
sulfur-containingcompoundsinwater.ThishappenswhenH2Sis
releasedinformofgas,itremainsintheatmosphereandspreadfor anaverageof18h,duringwhichtimeitisoxidizedtosulfurdioxide (SO2)andsulfuricacid(H2SO4).Also,becauseitisacolourlessand
flammablegasthatsmellslikerotteneggs,itcanbeperceivedat
lowconcentrationrangingfrom0.0005–0.3partpermillion(ppm).
However,athighconcentration,apersonmightlosethesenseof
smell,whichposesseriousdangertopeople’shealth,becauseof
false assumptionsthatitisnolongerpresent.Thiscanincrease
theirexposuretohigherconcentrationlevelthatmaycauseserious
health effects. Atconcentration 100–1000ppm, there is loss of
smell, serious respiration troubles, ocular irritation, loss of
consciousness,withimmediatedeathabove1000ppm[2,13].
Basically, therearetwomainutilizationsofbiogas.Itcanbe
eitherupgradedtoreachtheenergeticdensityofnaturalgasor
purifiedforchemicaltransformationprocesses.Biogasupgrading
consistsoftheadjustmentofCO2content,toincreasethecalorific
valueofthebiogastooptimallevel.Thus,biomethaneisthefinal
productwhichcomposesofCH4(95–99%)andCO2(1–5%),with
little or no trace of H2S and other compounds [14]. Biogas
purification involves the removal of harmful, toxic, and/or
unnecessary compounds such as H2S, N2, O2, Si, H, VOCs, CO,
andNH3.PurifiedbiogascontainsmostlyCH4andCO2withlow
contentofcontaminants.
Inrecentyears,severaltechnologieshavebeendevelopedfor
syngas cleaning, and their main differences are related to the
natureoftheoperation.Catalyticprocessesinvolvetheoxidation
ofH2Soversolidcatalysts,inordertoconvertittomolecularsulfur,
sulfide,thiosulfateandsulfate[9,15].Biologicalprocessesutilize bacteriasuchasThiobacillusandSulfolobusfamily,fortheoxidation ofH2Sintoelementalsulfurorsulfate[16].Thisprocessrequires
strict operational conditions to control pressure, temperature,
inputcomposition bacterialand energysources,etc.[17].Other
advancedmethodshadbeeninvestigatedanddevelopedsuchas
hydrateformationforH2Sseparation[18–22].Thesetechnologies
arefound tobeefficientfor biogas purificationand upgrading.
However,theystill needto beimproved for enhancing
perfor-manceandreducingoperationalcost[23].Inthiscontext,thereis
increasinginteresttodevelopnewlowcostandefficientmaterials
assorbentfor biogastreatment. Thishasledtoinvestigationof
industrial and agricultural wastes and sewage sludge [24] as
alternativesorbentsforH2Sremovalfrombiogas.
BiologicaldesulphurizationandbiofiltrationofH2Stechnology
mentionedaboveemployedspecializedmicroorganismstoreduce
thelevelofH2Sinbiogasbyconvertingittoelementalsulfurand
some sulphates, similar to the technique of addition of air or
oxygenintothedigestiontank.Thetechniqueisonthebasisof
lithautotrophicbacteriatouseH2SaselectrondonorandCO2as
carbon source, and also for the development of end-of-pipe
solutionsforbiogasupgrading[25,26].About4–6%ofair/oxygen
wasusedaselectronacceptorandprovidedtheenergyneededfor
lithotrophicgrowthinordertooxidizeH2S[25].Hydrateformation
process was firstly proposed by Yoon and Lee [27] who
Table1
Compositionandparametersofgasesfromdifferentorigins,impuritiesandtheirconsequences[5,12,20,43–47]. Parameters Unit Landfill
Gas Biogasfrom AD Dutch NaturalGas NorthSea NaturalGas
Impactonbiogasutilization MJ/ Nm3 16 23 31.6 40 Lowerheating value KWh/ Nm3 4.4 6.5 8.8 11 MJ/kg 12.3 20 38 47 Density Kg/ Nm3 1.3 1.1 0.8 0.84 Relative density – 1.1 0.9 0.6 0.63 Wobbeindex, upper MJ/ Nm3 18 27 43.7 55 Methane number – >130 >135 – 73 Methane(CH4) Vol-% 35–65 60–70 80–90 85–92 Heavy hydrocarbons Vol-% 0 0 9 9 Watervapour (H2O)
Vol% 1–5 1–5 – – Corrosionincompressors,gasstoragetanksandenginesduetoreactionwithH2S, NH3,CO2toformacids
Hydrogen Vol-% 0 0 – 0
Carbondioxide Vol-% 15–40 30–40 0.2–1.5 0.2–1.5 Decreasingcalorificvalue,anti-knockpropertiesofenginesandcorrosion Nitrogen,range Vol-% 15 0–0.5 14 0.3–1.0 Decreasingcalorificvalue,anti-knockpropertiesofenginesandcorrosion Oxygen Vol-% 1 0 – – Corrosion,foolingincavernstorage,riskofexplosion
Hydrogen sulfide
Ppm 0–100 0–4000 – 1.1–5.9 Corrosion,catalyticconverterpoison,emissionandhealthhazards.SO2,SO3are form
Ammonia (NH3)
Ppm 5 100 – 0 Emission,anti-knockpropertiesofenginesandcorrosionwhendissolved Totalchlorine
asCl"
Mg/
investigated clathrate phase equilibrium for the
water-phenol-carbondioxidesystembasedontheequilibriumpartitionofthe
componentsbetweengaseousphaseandthehydratephase.Kang
etel.[18]usedthisprincipletoworkongashydrateprocessforthe recoveryofCO2fromfluegas.AccordingtoTajimaetal.[21],the
basicmechanismoftheseparationprocessisaselectivepartition
of the target component between the hydrate phase and the
gaseousphase.Generally,thehydratephaseisstableunderhigh
pressure-low temperature conditions. This has attracted other
researcherstolookatthepossibilityofemployingthistechnology
to reduce CO2, H2S, and other contaminants from biogas and
syngasstreams.
This paper investigated the reactivity of calcium carbonate
(CaCO3)-basedsolid wastes(CCWs)fortheremovalofH2Sfrom
simulatedbiogasmatrix(H2Sinbiogas)andsyntheticwastegas
matrix(H2Sinair),usingtriphasicgas/liquid/solidprocessatroom
temperatureandatmosphericpressure.Themainobjectiveofthe
studywasthedevelopmentofnewandlowcostsorbentsforthe
treatment of H2S in the gas phase. The solid wastes, called
thereafterCCW-D(CalciumCarbonate–basedsolidWaste,sample
D) andCCW-S(CalciumCarbonate–basedsolidWaste,sampleS),
weretakenfromtwodifferentindustrialsitesoftheproductionof
sodiumcarbonateandsodiumbicarbonate,andwereanalyzedby
differentphysicochemicalmethods.Up-to-date,thesewastesare
notvalorizedforanyusefulmaterial.Inaddition,theycontaintrace
amountsofvariousmetalssuchasMg,Al,Fe,Si,Cl,Nawhichare
favourableforthefixationofacidgassuchashydrogensulfide[28–
30].Thesewastesarerich inCaCO3 andcanbereactiveforthe
fixationofacidgasfromthegasphase.Apurecommercialcalcite
(Calcite)andactivatedcarbon(AC)werealsousedasreferences.
TheperformanceofasinglesolidwasteorACwasalsocompared
with theirmixtures containing different weight ratios of solid
wastetoAC.
2. Materialsandmethods
2.1.Materials
TheinitialmaterialsusedinthisstudywereCCW-SandCCW-D
wastestakenfromtwodifferentindustrialsitesandwerebothin
powderedform.Theyweresievedtoeliminateparticleslargerthan
315
mm
anddriedat105!Cbeforeeachsorptiontest.Purecalcitepowder(CaCO3,>98wt%)fromFisherScientificand
ACpowderfromCARBIO12SA(C1220IG91,>1100m2/g,France)
werealsousedforH2Sremovaltest.TheACwasgrindedandsieved
torecoverparticlessmallerthan315
mm
forsorptiontest.Thesetwosorbentsarethereafterdesignated“Calcite”and“AC”forpure
calciteandactivatedcarbon,respectively.
2.2.Methods
Differentchemicalandphysicochemicaltechniqueswereused
forthecharacterizationsandanalysis ofthesorbentsbeforethe
H2Sremovaltest. Thermogravimetry-Differentialscanning
calo-rimetrycoupling(TG-DSC)wasperformedinaSDTQ600analyzer
(TA Instruments)witha heatingrate of5!C/minunderairflux
(100mL/min). X-ray diffraction (XRD) data of the solids were
collectedusingaPhillipsPanalyticalX’pertProMPDdiffractometer
withaCuKa(1.543Å)radiationsource.Thisdeviceworkswitha
currentof45kVandanintensityof40mA.Diffractionpeakshave
beenrecordedinthe2urangeof10to70!at0.042!persecond.
Specific surface area was determined by BET method, using a
MICROMETRICSTriStarII3020.Thetruedensityofthesorbents
was carried out by a helium pycnometer (ACCUPYC 1330,
Micrometrics).Particlesizedistributionwascarried outusinga
Mastersizer3000laserdiffractionparticlesizeanalyzer(Malvern
Instrument). Inductively coupled plasma coupled with atomic
emission spectrometryanalysis (ICP-AES) was performedusing
HORIBAJobinYvonUltima2,fortheelementalanalysis.Theionic
chromatography was performed with a Dionex ICS- 3000,
equippedwithaconductivitydetector.
2.3.Experimentalsetup
Thelaboratory-scalesetupwasdesignedandassembledforthe
adsorption test. It consists mainly of three parts: gas feeding,
adsorptionunit(glassreactor,200mL)andH2Sanalyzerasshown
inFig.1.Experimentswereconductedintwophases,firstlywith
syntheticwastegas(basedondriedaircontaining200ppmvH2S),
and secondly with synthetic biogas from Linde France S.A,
containingCH4 (64%), CO2 (31%),H2S(200ppmv)and balanced
withN2(around5%).Allexperimentswerecarried outatroom
temperature andatmospheric pressure.Foragivenexperiment,
100mLofdemineralizedwaterwas introducedintothereactor.
Thenthesyntheticwastegasorbiogascontaining200ppmvofH2S
was passedthroughthereactorattheflow rate of100mL/min
usingamassflowcontroller(SLA5851,Brooks1Instrument).The
reactorwaskeptunderstirringalongtheexperiment(600rpm).
FortheremovalofH2Sfromairmatrix,anelectrochemicaldetector
(GasAlertQuatro,fromBWTechnologies)wasusedtomonitorand
recordH2Sconcentrationatthereactoroutletevery5s,withthe
detectionlimitof0.1ppmv.FortheremovalofH2Sfromthebiogas
matrix,a
m-GC
(MyGC,Agilent)wasusedtoanalyzethegassampletakenfromtheoutletofthereactor.Infact,thesyntheticbiogas
contained any trace of oxygen sotheelectrochemical detector,
whichneedsO2forthemeasurement,couldnotbeusedforthisgas
matrix. WhenthewaterwascompletelysaturatedwithH2S(by
comparing the inlet and outlet concentration of H2S), 500 or
2500mgofsorbentwassetintothereactortostartthesorption
test.Theexperimentaltestswerevalidatedbyperformingatleast
twosets.Sodiumhydroxide(NaOH)wasusedtostrippedH2Sand
othercontaminantsfromtheexitgaseffluentfromthereactor,
beforedischargingitthroughtheceilingextractorfanfixeddirectly
abovethereactorroom.
In order to comparedifferent experimental conditions with
differentsorbents,thetotalaccumulatedquantityofH2Sinjected
tothereactoratagivenreactiontime(H2Sinput,mg)andthetotal
accumulated quantity of H2S fixed on the sorbent at a given
reactiontime(Accumulatedsorbent,mg)weredefinedand
calculat-edfromtheEqs.(1)and(2),respectively[28]
H2Sinput¼ PQM 106RTCintt ð1Þ Accumulatedsorbent¼ PQM 106RT½Cintt" Z t 0 Coutdt' ð2Þ
Where,Qistheinletflowrate(100mL/min),Misthemolecular
weightofH2S(34.06g/mol),CinistheinletconcentrationofH2S
(200ppmv),CoutistheoutletconcentrationofH2S(ppmv),tisthe
reactiontime(min),Pisthepressureofgas(1atm).
3. Resultsanddiscussion
3.1.Characterizationoftherawmaterials 3.1.1.ICP-AES
The elemental analysis of CCW-S and CCW-Dusing ICP-AES
technique is shown in Table 2. The commercial AC was not
analyzedsinceitcontainsmainlycarbon.ItisseenfromTable2
that the commercial calcite was practically pure, as expected.
CaCO3wasfoundtobethemaincomponentofCCW-SandCCW-D,
but other elements were also present which include different
metals, sulfur,and chlorine. CCW-Dcontained more impurities
thanCCW-SexceptforFe.Thecontentoforganiccompoundswas
negligibleforbothsolidwastes.
3.1.2.XRD
Fig. 2 shows XRD patterns of the initial sorbents. For the
commercialAC, twoweak andbroad peaksaround25 and45!
were observedwhich are characterized for amorphous
carbon-basedmaterial[29–31].For thecommercialcalcite,onlycalcite
wasdetectedasthecrystallinephase.CCW-ShadthesimilarXRD
patternthanthatofthepurecommercialcalcite,whichresulted
fromalonganddeepnaturalcarbonation.Well-crystallinecalcite
of rhombohedral structure was found to be the only major
crystalline phase in this waste. On the other hand, CCW-D
contained both calcite (rhombohedral structure) and aragonite
(orthorhombicstructure).Thepresenceofaragoniteisexplained
byafast-artificialcarbonationofCCW-D.Thecorrespondingmain
peaksareindicatedinFig.2. 3.1.3.Thermogravimetricanalysis(TG)
Thismeasuresweightchangesin amaterialasa functionof
temperatureand/ortimeunderacontrolledatmosphere.Fig.3(a)
presentstheweightlossofthesamplesduringaheatingtreatment
fromambienttemperatureto1000!Cwithaconstantheatingrate.
ForCaCO3-basedsorbents,aweightlossbetween610and800!C
wasobserved.Thiscorrespondstothedecarbonationofthecalcite
(Eq.(3)), which was highly endothermic(Fig. 3 (b)).From this
weightloss,thecontentofcalciumcarbonatecouldbedetermined
asshowninTable2above.Between200and610!C,asmalland
continuousweightlosswasalsoobservedforalltheCaCO3-based
sorbents. This may be due to the decomposition of other
carbonates at low contents (carbonatesof sodium,magnesium
etc.).
CaCO3!CaOþCO2 ð3Þ
For the commercial AC, a slight weight loss (13.4%) was
observed around 100!C which corresponds to the surface
Table2
Mainchemicalcompositionofresidues(CCW-S,CCW-D)andreferenceCalcite.
Materials Concentration(g/kg) Total(g/kg)
Ca CO32"b S Mg Na Fe Othersa
Calcite 434.5)1.2 576)0.4 – – – – – 1010.5)1.6 CCW-S 384.2)2.7 558)1.1 1.1)1.0 1.3)0.7 3.6)1.0 3.9)0.4 0.47 952.1)6.9 CCW-D 357.4)2.5 508)3.4 7.2)2.6 2.4)1.2 9.9)3.2 1.6)0.6 0.40 886.5)13.5 aP,Cu,Mn,Zn,Sn,Al,Cl,Si.
b DeterminedbythermogravimetricAnalysis.
Fig.2.XRDpatternsofthesorbentsshowingaragoniteandcalcite;C:calcite;A: aragonite.
humidityofAC(Fig.3(c)).Thehighquantityofhumidityofthis materialisexplainedbyitshighspecificsurfacearea(1100m2/g).A
weight loss was then observed above 380!C, due to the
combustion of AC. This combustion was strongly exothermic
which disturbed the furnace temperature, despite the small
amountofsampleusedforthis analysis(16.6mgonly).Asmall
weightlosswasalsorecordedabove650!Cwhichwasduetothe
gasification of coke deposition. Low ash content (<3%) was
observedforthisACindicatingitshighpurity.
3.1.4.Particlesizedistribution
Fig. 4 shows the particle size distribution of all the raw
materials before pretreatment. The commercial calcite had a
bimodaldistributionwithtwopeaksaround0.8and10
mm.
Theparticlesizedistributionofthissampleextendedfrom0.1toabout
45
mm.
TheCCW-Sand CCW-Dshowamultimodal distributionthatextended from0.1 to590
mm,
but themain peakwas alsofound around 10
mm.
Finally, the commercial AC presented amultimodal distribution with a broad peak. The main peak is
located around 210
mm.
Table 3 shows the values of thecharacteristicdiameters(d50andd90)ofthepowders.Theaverage
diameter(d50)isthesameorderofmagnitudeforthethreecalcium
carbonates,whichwassmallerthanthatofAC.
Partoftheobjectiveof thisresearchistovalorizetheentire
volumeofresidue.Toensurepropersuspensionofthesolidinthe
lab-scalereactorusedinthiswork(200mL)andthustoavoidthe
depositionofsolidsatthebottomofthereactor,thesolidswere
sifted.Atlaboratoryscale,thefractionofsolidusedwaslessthan
315
mm
aspreviouslymentioned.Thisfractioncorrespondsto99%byvolumeofthetworesidues.
3.1.5.Truedensity&specificsurfacearea
Thespecificsurfaceareaandthetruedensityofthesorbentsare
showninTable4.Alltheinitialcalciumcarbonate-basedsorbents
(Calcite,CCW-D,andCCW-S)hadaspecificsurfacearealowerthan
4g/m2,whilethatofACreached1098g/m2.Thetruedensityofthe
calciumcarbonate-basedsorbentswassimilartoeachother,while
that ofAC was smaller.Thisisexplained bythelowdensity of
carbon matrix of AC compared to calcium carbonate of other
sorbents.
3.2.SorptionreactivityintheremovalofH2Sinairmatrix
As stated earlier, H2S can be physically adsorbed, and
chemicallyabsorbedduetothepresenceofsomemetals,which
canactascatalystsorreagents,andalsothesurfacealkalinityof sorbent.Fig.5(a,b)presenttheresultsobtainedfortheremovalof
H2S(200ppmv)indryairmatrix.
InFig.5(a),whichpresentstheoutletconcentrationofH2Sasa
functionofthereactiontime,thepurecommercialcalcitehadthe
lowestreactivityforH2Sremoval.TheoutletconcentrationofH2S
wasmeasuredbythe
m-GC
forthisexperimentwhichexplainedthelimitednumberofanalyzedpoints,butitcanbeseenthatthe
suspensionofpurecalcitequicklylostitsreactivityabovethefirst
two hours of reaction. Among the two solid wastes, CCW-S
presentedlowerreactivitythanCCS-D.Bothsolidwastesquickly
decreasedtheconcentrationofH2Stoabout8ppmvattheoutletof
the reactor for the first min of reaction. Then the outlet
concentrationofH2Sprogressivelyincreasedandstagnatedaround
45–50ppmvfor CCW-Dand 150ppmv for CCW-S. Taking into
accountthelowspecificsurfaceareaofcalciumcarbonate-based
sorbents,itisexpectedthatthephysicaladsorptionofH2Sonthe
surfaceofthesesorbentsisnotenoughforexplainingthestagnant
period,observedforbothsolidwastes.Previousworkofourteam
explained this phenomenon by the catalytic oxidation of H2S,
catalyzedbyvariousmetalspresentinthesesolidwastes(Ca,Mg,
Feetc.)[32].Theacid-basicreactionofH2Swithsolublecarbonate
species (mainly CO32") in the aqueous solution plays also
importantrolefortheperformanceofthesesolidwastes.Inthe
caseof CCW-S,theoutletconcentrationof H2Sdecreasedagain
above 370min. It is supposed that a secondary dissolution of
CaCO3-based particles ofthis sorbent tookplace leading tothe
supplementaryavailabilityofactivespeciesfortheremovalofH2S.
The higher reactivity of CCW-D compared to CCW-S could be
explainedbythepresenceofaragonite(whichwasnotpresented
inCCW-S),aswellasthehighamountofimpurities ofCCW-D.
Aragoniteismoresolubleinwaterthancalcitewhichleadstohigh
availabilityofcarbonateanions fortheneutralizationof sulfide
species, whiletheimpurities (mostly metals andmetal oxides)
catalyzetheoxidationofsulfidespecieswithdissolvedoxygenas
oxidant.
For the commercial AC, the outlet concentration of H2S
decreasedtozeroand waskeptatthis valueforupto175min.
Thenitslowlyincreasedandreached25ppmvafter420min.So,AC
was more performant than the solid wastes during the first
minutesofreaction.ButitseemedthatACcontinuouslylostits
reactivity suggesting the physical adsorption of H2S as the
predominantphenomenon.On theotherhand,calcium
carbon-ate-basedwastesallowedkeepingconstantlytheremovalofH2S
thankstothecatalyticoxidationasthepredominantstep.
Fig.5(b)represents theresultsobtainedin Fig.5 (a)by the
accumulatedquantityofH2Sfedintothereactororfixedbythe
sorbents. This also allows comparing the performance of the
sorbents,and specificallythecalculationofthequantity ofH2S
Fig.4.ParticlesizedistributionsbyvolumeofCalcite,CCW-D,CCW-S,andActivatedcarbon(AC).
Table3
Valuesofthediametersd50andd90oftheinitialsorbents.
Materials d50(mm) d90(mm) Calcite 8.9 17 CCW-S 10.2 75.9 CCW-D 11.2 74.3 AC 24.9 108 Table4
Thesurfaceareaandthedensityofthesorbents.
Materials SurfaceArea(g/m2) AverageDensity(g/m3) Calcite 2.60 2.73
CCW-S 3.23 2.76
CCW-D 3.98 2.70
fixedby thesorbentata givenreaction time.The curveofthe
accumulatedquantityofH2SfixedbyACwassuperposedormostly
closetothecurveoftheaccumulatedquantityofH2Sintroducedto
thereactorduringthereactiontime.So,mostH2Sintroducedtothe
reactorwasfixedbythissorbent.Bythesameanalysis,CCW-Dwas
more performant than CCW-S and pure calcite. Basing onthe
resultsin Fig.5(a,b),only thecommercialAC and CCW-Dwere
selectedforthestudyofH2Sremovalinbiogasmatrixbelow.
3.3.SorptionreactivityintheremovalofH2Sinbiogasmatrix
ThesyntheticbiogascontainingwasCH4(64%),CO2(31%),H2S
(200ppmv),andbalancedwithN2(around5%),andwasobtained
fromLindeFranceSA.TheremovalofH2Sinthisbiogasmatrixwas
performedwithtwosorbentsACandCCW-D.Fig.6(a)showsthe
outlet concentration of H2S obtained with 0.5 or 2.5g of the
individual sorbents. Note that using the
m-GC
analysis withmanualinjection,therewaslessanalysispointcomparedtothe
electrochemicaldetector.Inallcase,CCW-Dhadlowerreactivity
thanAC.ForbothquantitiesofCCW-Dof0.5and2.5g,theprofileof theoutletconcentrationofH2Swasquitesimilar.Thisisexplained
bythefactthatthesyntheticbiogasdidnotcontainoxygensono
oxidationofsulfidespeciestookplace.DissolvedH2Scouldonlybe
neutralized by carbonate anions (CO32") coming from the
dissolutionofcalciumcarbonateandthisisslowbecausecalcium
carbonate (in both calcite and aragonite forms) haslow water
solubility. The physisorption of H2S must be low in this case,
becauseCCW-Dhad verylowspecificsurfaceareaandwasnot
porous. Thus, the outlet concentration of H2S progressively
increase and was closetotheinlet valueafter540minof test.
Incombinationwiththeresultsintheairmatrix,wecanseethat
the catalytic oxidation is predominant when using calcium
carbonatewastesas sorbents.In fact, theanalysis oftheliquid
phase recovered after test in the air matrix evidenced the
formationofoxidationproducts(sulfate,sulfite,andthiosulfate).
Thiswaspreviouslydetailed[32].Ontheotherhand,
physisorp-tion,whichisfavourablebyhighspecificsurfaceareasuchasthe
caseofAC usedin thiswork(1098m2/g),mustbepredominant
when usingAC as sorbent.Thisis not (orless)affectedby the
presenceorabsenceofoxygeninthegasmatrix.Theincreaseofthe
quantityofACfrom0.5to2.5gthusallowedstronglydecreasing
theoutletconcentrationofH2Sduringthereaction.After540min,
theoutletconcentrationofH2Swas157and27ppmvusing0.5and
2.5gofAC,respectively.Fig.6(b)representstheresultsobtainedin
Fig.6(a)by theaccumulatedquantity ofH2S. The effectof gas
matrixontheperformanceoftheprocesswasagainevidenced.
Fig.5.ExperimentalresultsinH2Spassingoverthesorbents(AC,CALCITE,CCW-DandCCW-S)fortheremovalofH2S(200ppmv)dilutedinairwithexperimentalconditions ofroomtemperatureandpressure,500mgofsorbent,100mL/minofinputgas,100mLofwater:(a):OutletconcentrationofH2S;(b):AccumulatedquantityofH2Sfixedon sorbent.
When0.5gACwasused,theperformancewashigherwithair
matrix(11mgofH2Saccumulatedonthesorbentafter400minof
reactiontime,Fig.5(b))thanwithbiogasmatrix(versus6mgof
H2Saccumulatedonthesorbentafter400minofreactiontime,
Fig.6(b)).Infact,inFig.5(a),astagnantperiodatzeroppmvfor
175minwasobservedwiththeairmatrixbutitwasnotthecase
withthebiogasmatrix.Takingintoaccountthecompositionofthe
syntheticbiogasusedinthiswork,mostlywiththepresenceofCH4
andCO2,itissupposedthatthesetwomoleculeshadcompetitive
adsorptionagainstH2S.Thus,itdecreasedtheperformanceofACin
thebiogasmatrixforthefixationofH2S.Thecompositionofthe
biogaswasallanalyzedduringthereaction.Thesignificantchange
ofitscomposition(forCO2,CH4andN2)wasnotobserved,probably
due tohighcontentsofthesegases.ThereactivityofACforthe
adsorption of CO2 and CH4 should befurtherdone in orderto
evidencetheirimpactonthereactivityofACforH2Sfixation.
3.4.Sorptionreactivityofmixedsorbentsinbiogasmatrix
The results obtained above withthe single sorbents (AC or
calciumcarbonate-basedmaterial)showthateachsorbenthadits
own interaction with H2S. For AC, the physisorption was
predominant.Forcalciumcarbonate-basedmaterial,thecatalytic
oxidation,enhancedbyacid-basicreaction,mainlytookplace.Itis
usefultofocusontheinvestigationof thereactivityofdifferent
mixtureofACandCCW-DfortheremovalofH2Sfromthebiogas
matrix.Forthis,onlyonesorbentquantitywasused(2.5g).Five
sorbentmixturesweretestedwiththeweightratioofACtoCCW-D
equalto1:1;1:2;2:1,1:3;and3:1,bykeepingthetotalweightof 2.5g(Table5).Fig.7(a)showstheoutletconcentrationofH2Sasa
functionofthereactiontimeobtainedwiththesemixtures.The
resultsobtainedwiththeindividualCCW-DorACalsoincludedfor
comparison. Fig.7(b) represents the resultsof Fig. 7(a) bythe
accumulatedquantityofH2S.
InFig.7(a),itcanbeseenthattheincreaseoftheACcontentled
totheimprovementoftheperformanceofthemixture.Thusthe
order of the reactivity of these mixtures could be classed as
following: AC3:CCW-D1>AC2:CCW-D1>AC1:CCW-D1*
individ-ualAC>AC1:CCW-D2*AC1:CCW-D3>individualCCW-D.Infact,
after540minofreactiontime,thequantityofH2Saccumulatedin
eachsorbentwas:15mgforAC3:CCW-D1,14.5mgfor
AC2:CCW-D1,14mgforAC1:CCW-D1andACalone,10.5mgforAC1:CCW-D2,
10mgforAC1:CCW-D3and2mgforCCW-Dalone.At30minof
reaction,theoutletconcentrationofH2Swasalready5.6–7.3ppmv,
Fig.6.ExperimentalresultsinH2Spassingoverthesorbents(ACandCCW-D)fortheremovalofH2S(200ppmv)inbiogasmatrix(CH4=64%,CO2=31%,N2=5%),with experimentalconditionsofroomtemperatureandpressure,2500and500mgofsorbent,100mL/minofinputgas,100mLofwater:(a):OutletconcentrationofH2S;(b): AccumulatedquantityofH2Sfixedonsorbent.
Table5
CompositionofdifferentmixturesofACandCCW-Dusedinthiswork. Designation WeightratioofACtoCCW-S Totalweight,g
AC3:CCW-D1 3:1 2.5
AC2:CCW-D1 2:1 2.5
AC1:CCW-D1 1:1 2.5
AC1:CCW-D2 1:2 2.5
whichprogressivelyroseto123–130ppmat540min.An
impor-tant change in the performance was obtainedwhen using the
mixture containing 50wt% of AC (AC1:CCW-D1). This mixture
reachedthepractically-completeremovalofH2Sforatleastthe
first180min.Thenthismixtureslowlylostitsreactivitybutitwas
comparabletotheindividualACatthesametotalweightof2.5g.
TheincreaseoftheACcontentto66.7%(AC2:CCW-D1)and75%
(AC3:CCW-D1) allowed still increasing the performance of the
process.At75%ofAC,theoutletconcentrationofH2Swaskept
closetozeroformorethan480minandwasonlyof5.6ppmvafter
540min. This mixture was much more performant than the
individualACorCCW-D,whichsuggestsaveryinterestingsynergy
effectofthemixturefortheremovalofH2S.
3.5.Discussion
Two solid wastes (CCW-D and CCW-S) containing mainly
calciumcarbonatewereinvestigatedintheremovalofH2Sfrom
thegasphase.Undertheairmatrix,theywerereactivebecauseof
the availability of basic species (carbonate anions) for the
neutralization of sulfide species, and the catalytic activity of
variousmetalsinitiallypresentinthesewastes.Theyweremore
reactive than a pure commercial calcite. However, under the
reducingatmosphereofbiogas,withoutoxygen,thesesolidwastes
had only low reactivity, explained by the absenceof oxidation
reactions.
ThecommercialACusedasreferenceinthisworkwasfoundto
beefficientfortheremovalofH2Sfrombothairandbiogasmatrix.
However,whenACandCCW-Dwerecombined,theperformance
washighlyimprovedcomparedtotheindividualsorbentsinthe
biogasmatrix. Thesynergyeffectof this combinationmight be
explainedbythephysico-chemicalpropertiesofthetwosorbents.
CCW-D,containingmainlycalciumcarbonate,hashighbasicity.
ThisallowedacceleratingthedissolutionanddissociationofH2S
fromthegasphasetotheliquidphase.Butthiswasnotenoughto
maintaintheabatementofH2Swiththereactiontimeunderthe
biogasmatrixbecausedissolvedsulfidespecieswereaccumulated
inthereactor(withoutoxidationreactions).Thecombinationwith
AC allowed the fixation of dissolved sulfide species and thus
improvedthefixationrateofH2S.
In the aqueous solution, H2S can simply exist under the
dissolvedform(H2Saq)orcanbedissociatedtoHS"andS2",asa
functionofthepH.ThefavourableeffectofCCW-Donthereactivity of AC suggests that the dissociativeforms of sulfides HS",S2"
Fig.7. ExperimentalresultsinH2Spassingovermixturesofsorbents(AC:CCW-D)fortheremovalofH2S(200ppmv)inbiogasmatrix(CH4=64%,CO2=31%,N2=5%),with experimentalconditionsofroomtemperatureandpressure,2500mgofsorbents(weightratioofACtoCCW-Dof1:1,2:1,1:2,1:3and3:1),100mL/minofinputgas,100mLof water:(a):OutletconcentrationofH2S;(b):AccumulatedquantityofH2Sfixedonsorbent.
mightbepreferentiallyadsorbedonthesurfaceofAC.Inthiscase,
thebasicityofthesolidwasteisusefulfortheneutralizationof
protonscomingfromthedissociationofH2S.
InFig.7(a),themixtureAC3:CW-D1(composedof1.875gofAC
and0.625gofCCW-D)andthemixtureAC2:CW-D1(composedof
1.667gofACand0.833gofCCW-D)showhigherreactivitythan
theindividualAC, atthesame sorbentweightused(2.5g). The
mixtureAC:CW-D1(composedof1.25gofACand1.25gofCCW-D)
hadthecomparablereactivitythantheindividualAC.Takinginto
account the high cost of commercial AC compared to calcium
carbonate-basedwaste,itisveryinterestingtocombinethesetwo
materialsforthedesignofnewperformanceandlowcostsorbent.
Moreimportantly,consideringtheimportantannualproductionof
sodiumcarbonateandsodiumbicarbonate(about60milliontons
worldwide in2014[33]), largequantities ofcalcium
carbonate-basedwastesaregeneratedannually.Thehighavailability,thelow
costandthegoodreactivityforthefixationofacidgasofthese
wastes areimportantcriteriafor theviabilityof theprocess. In
addition,up-to-date,thesewastesarenotvalorizedforanyuseful
material. The biogas purification may open new route for the
valorizationofthesewastes,whichhavebeencreatingproblems
forammoniasodaplants[34–36].
Fig.8showstheevolutionoftheconcentrationofCH4,CO2,N2
andH2SwiththereactiontimeusingthemixtureAC3:CCW-D1in
thebiogasmatrix. BothCH4 and CO2 concentrationlevelswere
ratherstablethroughouttheexperiment,whichwasverypositive
forthestudy.Itmeansthatthesorbentsdevelopedinthisworkhas
highselectivityandcompatibilitytowardspolarcompounds,such
asH2SandthereforemoreappropriateforH2Sremoval.
Inthiswork,theonlypretreatmentappliedtothesolidwastes
wasthedryingat105!Ctoremovemoisture,becauseofthenature
ofthewastematerialstorage,whichismostlystoredoutsidewith
orwithoutcovers.Itispossibletoincreasetheperformanceofthe
sorbentbythermallytreating itathighertemperature.Thermal
treatment is known as an efficient treatment for desorbing
moleculesadsorbedonthesurfaceofagivensolidaswellasfor
combustionof eventualorganicmoleculespresentin theinitial
sorbents.Inthecaseofcalciumcarbonate-basedsorbent,athigh
temperature (above 610!C), thedecarbonation takes place and
increasesthebasicityandsothereactivityofthismaterial.Further
workwillfocusontheinfluenceofthethermaltreatmentonthe
reactivityofcalciumcarbonate-basedsorbents.
Actually,therearedifferentnovelaspectsofthepresentstudy.
First, H2Sremoval test was carried out with two differentgas
matrix,which representgaseous effluent(airmatrix)or biogas.
ThisallowsidentifyingtheimpactofCO2(andCH4)onthefixation
ofH2Sunderthesimilaroperational conditions.Theuseofthe
triphasicgas/liquid/solid processfor gascleaningseemed tobe
alsoanoveltyofthiswork.ThisacceleratesthetransferofH2Sfrom
gastoliquidphasewhichisfavourableforthecontactofsulfide
specieswiththesorbents.Finally,animportantaspectofthestudy
istheuseofmixturesofactivatedcarbonwithcalciumcarbonate
wasteasnewsorbentsforsorptiontest.Thesemixtureshadhigher
reactivity than AC or solid wastealone. It is believed that the
reductioninthequantityof AC,thatwillberequiredfor biogas
treatment as a result of substituting it with low cost waste
materialsassorbentswillleadtoreductionincostofAC,thereby
leading to overall reduction biogas purification and upgrading
costs.
Theresultsofthecurrentstudyagreedwithearlierworksofour
teamandotherresearchersonH2Sremovalfromgasphase.Galera
Martinez [32] worked on valorization of industrial carbonate
residuesforthetreatmentofhydrogensulfideingaseffluents,and
explained the catalytic oxidation of H2S, catalyzed by various
metalspresentinthesesolidwastes(Ca,Mg,Feetc.).Also,Stita
etal.[37]studiedmetal-dopedapatiticcalciumphosphatesforthe
removalofhydrogen sulfide fromgasphase, while Pham Xuan
etal.[28]studiedonthevalorizationofcalciumcarbonate-based
solidwastesforthetreatmentofhydrogensulfidefromgasphase.
Lastly,PhamMinhetal.[38]conductedthesimilarresearchbased
on calcium phosphate-based materials starting from calcium
carbonateand orthophosphoric acidfor theremovalof lead(II)
from an aqueous solution. In addition to these, Lasocki [39]
investigatedbiogastreatmentbyremovalofhydrogensulfideand
carbondioxideatlaboratory-scale,usingbothabsorptionsinliquid
phase (barbotage process) and solid bed of reagent. Also, H2S
removalfrombiogasonsludge-derived adsorbentswas
investi-gatedbyotherresearchers[24,40–42],usingsimilartechniques.
Theseearlierinvestigationsandotherstudiesintheliteraturewere basedsolelyonsyntheticwastegasmatrix(H2Sinair)only,except
Fig.8.EvolutionoftheconcentrationofCH4,CO2,N2andH2SattheoutletofthereactorasafunctionofthereactiontimewhenusingthemixtureAC3:CCW-D1inthebiogas matrix.
Ortiz[24],whousedsimulatedbiogas.Thismightbeduetothefact
thatCO2mightcompeteagainstH2Sinadsorptiondependingon
theporousstructureoftheadsorbentandthealkaliconstituents
sincetheyarebothacidgases.Whereasthisstudywasbasedon
bothsimulatedbiogasmatrix(H2Sinbiogas)andsyntheticwaste
gasmatrix(H2Sinair),usingtriphasicgas/liquid/solidprocessat
roomtemperatureandatmosphericpressure.Asstatedearlierand
showninFig.8,bothCH4andCO2concentrationlevelswererather
stablethroughouttheexperiment,whichwasverypositiveforthe
study.
4. Conclusions
Inthis study, AC wasfoundtobethe mostreactivefor H2S
removalfromthegasphase,followedbyCCW-D,CCW-Sandpure
commercialcalcite.Physisorptionwas predominantwhenusing
AC while acid-basic reaction and catalytic oxidation were
predominant with calcium carbonate-based sorbents.
Conse-quently, AC slowly lost its performance while the calcium
carbonate-basedwastes keptconstantlytheirreactivity(bythe
catalyticoxidation).Inthebiogasmatrix,onlyACandCCW-Dwere
investigated. CCW-D quickly lost its reactivity because no
oxidationreactiontookplace, whichwasdue totheabsenceof
oxygen in the biogas matrix used. AC showed again its high
performanceforthephysisorptionofH2S.Butitsreactivitywas
lowerin thebiogasmatrixthan in theairmatrix (atthesame
sorbentweightusedof0.5g), whichcouldbeexplainedbythe
competitive adsorption of CH4 and CO2 present in the biogas
matrix.
ThereactivityofdifferentmixturesofACandCCW-Dwerealso
investigatedinthebiogasmatrix.Atthesamesorbentloadof2.5g
withat least50wt%of AC,a synergyeffectofthemixturewas
observed for the removal of H2S, compared to the individual
sorbent.Thisbeneficialeffectwasexplainedbythecombinationof
the high basicity of CCW-D (by the dissolution of calcium
carbonate)andthehighspecificsurfaceareaofAC,availablefor
thefixationofthesulfide species.Therefore,calciumcarbonate
wastecanbeusedasco-sorbentwithACinthetriphasicprocessfor
theremovalofH2Sfromthegasphase.Thisopensnewperspective
forthevalorizationofthiskindofsolidwastes,inparticularlyfor
thepurificationofbiogas.
Acknowledgements
ThefirstauthoracknowledgesthesupportfromtheSchoolof
Civil Engineering,UniversityCollegeDublin, tuition scholarship
support from Student Universal Support Ireland (SUSI) and
Universite’ de Toulouse, Mines Albi, CNRS UMR 5302, Centre
RAPSODEE,CampusJarlard, Albi,F-81013cedex 09,France.The
authorgratefullyacknowledgescolleaguesand technicalstaffat
RAPSODECenterfortechnicalhelp.
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